1. Field of the Invention
The present invention relates to a magnetic memory for storing data in magnetoresistive effect elements.
2. Related Background Art
An MRAM (Magnetic Random Access Memory) with magnetoresistive effect elements is presently drawing attention as a storage device used in information processing apparatus such as computers and communication equipment. The MRAM is a type of memory that stores data by magnetism, and is thus free of such inconvenience that information is lost with power discontinuity, different from the DRAM (Dynamic Random Access Memory) and SRAM (Static RAM) being volatile memories. In addition, the MRAM is much superior in access speed, reliability, power consumption, etc. to nonvolatile storage means such as the conventional flash EEPROM (Electronically Erasable and Programmable Read Only Memory) and hard disk drives. Therefore, the MRAM holds the potential to replace all the function of the volatile memories such as the DRAM and SRAM and the function of the nonvolatile storage means such as the flash EEPROM and hard disk drives. Nowadays, there are rapidly ongoing efforts to develop information equipment aiming at so-called ubiquitous computing to enable information processing anytime and anywhere, and the MRAM is expected to play a role as a key device in such information equipment.
The magnetoresistive effect elements are elements making use of the Giant Magneto-Resistive (GMR) effect or the Tunneling Magneto-Resistive (TMR) effect, for example. The GMR effect is a phenomenon in which the resistance of ferromagnetic layers in a direction perpendicular to a stack direction thereof varies according to a relative angle between magnetization directions of two ferromagnetic layers with a nonmagnetic layer in between. Namely, in the case of the giant magnetoresistive effect element (GMR element) making use of the GMR effect, the resistance of the ferromagnetic layers becomes minimum when the magnetization directions of the two ferromagnetic layers are parallel to each other, and the resistance is maximum when they are antiparallel. The TMR effect is a phenomenon in which the resistance between two ferromagnetic layers varies according to a relative angle between magnetization directions of two ferromagnetic layers with a thin insulating layer in between. Namely, in the case of the tunneling magnetoresistive effect element (TMR element) making use of the TMR effect, the resistance between the ferromagnetic layers becomes minimum when the magnetization directions of the two ferromagnetic layers are parallel, and the resistance becomes maximum when they are antiparallel. The MRAM with such magnetoresistive effect elements is able to store binary data (0 or 1) by a state of the magnetization directions between two ferromagnetic layers (parallel or antiparallel). Then the binary data can be read out of the magnetoresistive effect element by measuring the resistance based on the state of the magnetization directions.
Particularly, the TMR element among these magnetoresistive effect elements can achieve a large resistance change rate, e.g., 40% or more. Since the TMR element has a relatively high resistance, it can be readily combined with a semiconductor device such as a MOS-FET. Therefore, stored data can be stably read out by a relatively small current and there are thus hopes for increase in storage capacity and for improvement in operation speed. An MRAM using the TMR effect is a magnetic memory disclosed, for example, in Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-153182).